v3.25.1
Label Element Value
Risk Return Abstract rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName STATE STREET INSTITUTIONAL INVESTMENT TRUST
Prospectus Date rr_ProspectusDate Apr. 30, 2025
State Street Small_Mid Cap Equity Index Fund  
Risk Return Abstract rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;text-transform:uppercase;">State Street Small/Mid Cap Equity Index Fund</span>
Objective [Heading] rr_ObjectiveHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Investment Objective</span>
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The State Street Small/Mid Cap Equity Index Fund (the “Small/Mid Cap Equity Index Fund”or the “Fund”) seeks to provide investment results that, before expenses, correspond generally to the total return of an index that tracks the performance of mid-to small- capitalization exchange traded U.S. equity securities.
Expense [Heading] rr_ExpenseHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Fees and Expenses of the Fund</span>
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock The tables below describe the fees and expenses that you may pay if you buy, hold, and sell shares of the Fund (“Fund Shares”). You may pay other fees, such as brokerage commissions and other fees to financial intermediaries which are not reflected in the tables and examples below.You may qualify for sales charge discounts on purchases of Class A shares if you agree to invest in the future, or if you and your family currently invest, at least $50,000 in State Street Funds that offer Class A shares. More information about these and other discounts is available from your financial intermediary (“Financial Intermediary”)and in Choosing a Share Class – Reducing Your Class A Sales Charge on page 63 of the Fund's Prospectus. The expenses shown in the table and the Example reflect the expenses of both the Fund and the Fund's proportionate share of the expenses of State Street Small/Mid Cap Equity Index Portfolio (the “Small/Mid Cap Equity Index Portfolio”or sometimes referred to in context as the “Portfolio”).
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Shareholder Fees </span><span style="color:#000000;font-family:Arial;font-size:10.02pt;">(fees paid directly from your investment)</span>
Operating Expenses Caption [Text] rr_OperatingExpensesCaption <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Annual Fund Operating Expenses </span><span style="color:#000000;font-family:Arial;font-size:10.02pt;">(expenses that you pay each year as a percentage of the value of your investment)</span>
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination <span style="font-family:Arial;font-size:9.02pt;">April 30, 2026</span>
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Portfolio Turnover:</span>
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over”its portfolio). As a shareholder of the Small/Mid Cap Equity Index Portfolio, the Fund bears its ratable share of the transaction costs associated with the portfolio turnover of the Portfolio. A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund Shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the Fund's performance. During the most recent fiscal year, the Fund's portfolio turnover rate was 24% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 24.00%
Expenses Deferred Charges [Text Block] rr_ExpensesDeferredChargesTextBlock A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.
Expense Breakpoint Discounts [Text] rr_ExpenseBreakpointDiscounts <span style="color:#000000;font-family:Arial;font-size:10.02pt;">You may qualify for sales charge discounts on purchases </span><span style="color:#000000;font-family:Arial;font-size:10.02pt;margin-left:0%;">of Class A shares if you agree to invest in the future, or if you and your family currently invest, at least $</span><span style="color:#000000;font-family:Arial;font-size:10.02pt;">50,000</span><span style="color:#000000;font-family:Arial;font-size:10.02pt;"> in State </span><span style="color:#000000;font-family:Arial;font-size:10.02pt;margin-left:0%;">Street Funds that offer Class A shares.</span>
Expense Breakpoint, Minimum Investment Required [Amount] rr_ExpenseBreakpointMinimumInvestmentRequiredAmount $ 50,000
Expenses Restated to Reflect Current [Text] rr_ExpensesRestatedToReflectCurrent <span style="color:#000000;font-family:Arial;font-size:9.02pt;">Other Expenses have been restated to reflect current fees.</span>
Expense Example [Heading] rr_ExpenseExampleHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Example:</span>
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated, and then sell or hold all of your Fund Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund's operating expenses remain the same. The calculation of costs for the one-year period takes into account the effect of any current contractual fee waivers and/or reimbursements; and the calculation of costs for the remaining periods takes such fee waivers and/or reimbursements into account only for the first year of each such period. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Principal Investment Strategies</span>
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock The Fund uses a passive management strategy designed to track the performance of the Russell Small Cap Completeness Index (the “Index”). The Index is a float-adjusted, market capitalization index which measures the performance of the Russell 3000® Index companies excluding S&P 500® constituents. The Index is constructed to provide a comprehensive and unbiased barometer of the extended broad market beyond the S&P 500 exposure. The Index is completely reconstituted annually to ensure new and growing equities are included. As of February 28, 2025, the market capitalization of the companies included in the Index ranged from $1.21 million to $122.09 billion. As of February 28, 2025, the Index comprised 2,457 securities. As of February 28, 2025, a significant portion of the Index comprised companies in the information technology, healthcare, industrials, financials and consumer discretionary sectors, although this may change from time to time. The Fund is not managed according to traditional methods of “active”investment management, which involve the buying and selling of securities based upon economic, financial and market analysis and investment judgment. Instead, the Fund, using a “passive”or “indexing”investment approach, seeks to provide investment results that, before expenses, correspond generally to the total return of the Index. The Fund generally intends to invest in all stocks comprising the Index in approximate proportion to their weightings in the Index. However, under various circumstances, it may not be possible or practicable to purchase all stocks in those weightings. In those circumstances, the Fund may purchase a sample of the stocks in the Index in proportions expected by SSGA Funds Management, Inc. (“SSGA FM”or the “Adviser”), the investment adviser to the Fund, to match generally the performance of the Index as a whole. In addition, from time to time, stocks are added to or removed from the Index. The Fund may sell securities that are represented in the Index, or purchase securities that are not yet represented in the Index, in anticipation of their removal from or addition to the Index. Under normal market conditions, the Fund invests at least 80% of its net assets (plus borrowings, if any) in stocks in the Index. The Fund will provide shareholders with at least sixty (60) days' notice prior to any change in this 80% investment policy. The notional value of the Fund's investments in derivatives or other synthetic instruments that provide exposures comparable, in the judgment of the Adviser, to investments in the Index may be counted toward satisfaction of this 80% policy. In addition, the Fund may invest in equity securities that are not included in the Index, cash and cash equivalents or money market instruments, such as repurchase agreements and money market funds (including money market funds advised by the Adviser). The Fund may purchase or sell futures contracts on the Index, or options on those futures, in lieu of investing directly in the stocks making up the Index. The Fund might do so, for example, in order to increase its investment exposure pending investment of cash in the stocks comprising the Index. Alternatively, the Fund might use futures or options on futures to reduce its investment exposure in situations where it intends to sell a portion of the stocks in its portfolio but the sale has not yet been completed. The Fund may also enter into other derivatives transactions, including the use of options or swap transactions relating to the Index or any securities comprising the Index, in lieu of investing directly in the stocks making up the Index. The Fund may, to the extent permitted by applicable law, invest in shares of other mutual funds whose investment objectives and policies are similar to those of the Fund (including funds advised by the Adviser). The Fund seeks to achieve its investment objective by investing substantially all of its investable assets in the State Street Small/Mid Cap Equity Index Portfolio, a separate series of the State Street Institutional Investment Trust, with an identical investment objective as the Fund and that has substantially identical investment policies, strategies and risks, to the Fund. When the Fund invests in this “master-feeder”structure, the Fund's only investments are shares of the Portfolio and it participates in the investment returns achieved by the Portfolio. The Index is sponsored by FTSE Russell (the “Index Provider”), which is not affiliated with the Fund or the Adviser. The Index Provider determines the composition of the Index, relative weightings of the securities in the Index and publishes information regarding the market value of the Index.
Risk [Heading] rr_RiskHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Principal Risks</span>
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Performance</span>
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The bar chart and table below provide some indication of the risks of investing in the Fund by illustrating the variability of the Fund's returns from year-to-year and by showing how the Fund's average annual returns for the periods indicated compared with that of (i) a broad measure of market performance and (ii) the Index. The bar chart shows how the Class A shares' returns have varied for each full calendar year shown. The Fund's past performance does not necessarily indicate how the Fund will perform in the future.Current performance information for the Fund is available toll free by calling (800) 997-7327 or by visiting our website at  www.ssga.com.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns <span style="color:#000000;font-family:Arial;font-size:10.02pt;margin-left:0%;">The bar chart and table below provide some indication of the risks of investing in the Fund by illustrating the variability of the Fund's returns from year-to-year and by showing how the Fund's average annual returns for the periods indicated compared with that of (i) a broad measure of market performance and (ii) the Index. </span>
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone <span style="color:#000000;font-family:Arial;font-size:10.02pt;">(800) 997-7327</span>
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress <span style="color:#000000;font-family:Arial;font-size:10.02pt;"> www.ssga.com</span>
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture <span style="color:#000000;font-family:Arial;font-size:10.02pt;">The Fund's past performance does not necessarily</span><span style="color:#000000;font-family:Arial;font-size:10.02pt;margin-left:0%;"> indicate how the Fund will perform in the future.</span>
Bar Chart [Heading] rr_BarChartHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Annual Total Returns </span><span style="color:#000000;font-family:Arial;font-size:10.02pt;">(years ended 12/31)</span>
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock ReturnsQuarter/YearHighest Quarterly Return30.41%Q2 2020Lowest Quarterly Return-28.02%Q1 2020
Performance Table Heading rr_PerformanceTableHeading <span style="font-family:Arial;font-size:10.02pt;font-weight:bold;">Average Annual Total Returns </span><span style="font-family:Arial;font-size:10.02pt;">(for periods ended 12/31/24)</span>
Performance Table Does Reflect Sales Loads rr_PerformanceTableDoesReflectSalesLoads <span style="color:#000000;font-family:Arial;font-size:10.02pt;">Except for differences in returns resulting from differences in fees, expenses, </span><span style="color:#000000;font-family:Arial;font-size:10.02pt;margin-left:0%;">and sales charges (as applicable), all share classes would have substantially similar returns because all share classes </span><span style="color:#000000;font-family:Arial;font-size:10.02pt;">invest in the same portfolio of securities.</span>
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate <span style="color:#000000;font-family:Arial;font-size:10.02pt;margin-left:0%;">After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. </span>
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred <span style="color:#000000;font-family:Arial;font-size:10.02pt;">After-tax returns depend on an investor's tax situation and may differ from those </span><span style="color:#000000;font-family:Arial;font-size:10.02pt;margin-left:0%;">shown, and after-tax returns are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (</span><span style="color:#000000;font-family:Arial;font-size:10.02pt;">“IRAs”).</span>
Performance Table One Class of after Tax Shown [Text] rr_PerformanceTableOneClassOfAfterTaxShown <span style="color:#000000;font-family:Arial;font-size:10.02pt;">After tax returns are shown only for Class A and after </span><span style="color:#000000;font-family:Arial;font-size:10.02pt;margin-left:0%;">tax returns will vary for other share classes.</span>
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. After-tax returns depend on an investor's tax situation and may differ from those shown, and after-tax returns are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (“IRAs”).After tax returns are shown only for Class A and after tax returns will vary for other share classes.Except for differences in returns resulting from differences in fees, expenses, and sales charges (as applicable), all share classes would have substantially similar returns because all share classes invest in the same portfolio of securities.
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock The Fund will make updated performance information, including its current net asset value, available at the Fund's website:  www.ssga.com.
State Street Small_Mid Cap Equity Index Fund | Risk Lose Money [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock You could lose money by investing in the Fund.
State Street Small_Mid Cap Equity Index Fund | Risk Not Insured Depository Institution [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock An investment in the Fund is subject to investment risks, including possible loss of principal, is not a deposit in a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation (“FDIC”) or any other government agency.
State Street Small_Mid Cap Equity Index Fund | Market Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Market Risk: The Fund's investments are subject to changes in general economic conditions, general market fluctuations and the risks inherent in investment in securities markets. Investment markets can be volatile and prices of investments can change substantially due to various factors including, but not limited to, economic growth or recession, changes in interest rates, inflation, changes in the actual or perceived creditworthiness of issuers, and general market liquidity. The Fund is subject to the risk that geopolitical events will disrupt securities markets and adversely affect global economies and markets. Local, regional or global events such as war, military conflicts, acts of terrorism,trade policy changes or disputes, the threat or actual imposition of tariffs, natural disasters, the spread of infectious illness or other public health issues, or other events could have a significant impact on the Fund and its investments.
State Street Small_Mid Cap Equity Index Fund | Equity Investing Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Equity Investing Risk: The market prices of equity securities owned by the Fund may go up or down, sometimes rapidly or unpredictably. The value of a security may decline for a number of reasons that may directly relate to the issuer and also may decline due to general industry or market conditions that are not specifically related to a particular company. In addition, equity markets tend to move in cycles, which may cause stock prices to fall over short or extended periods of time.
State Street Small_Mid Cap Equity Index Fund | Small Mid and Micro Capitalization Securities Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Small-, Mid-, and Micro-Capitalization Securities Risk: The securities of small-, mid- and micro-capitalization companies may be more volatile and may involve more risk than the securities of larger companies. These companies may have limited product lines, markets or financial resources, may lack the competitive strength of larger companies, and may depend on a few key employees. In addition, these companies may have been recently organized and may have little or no track record of success. The securities of smaller companies may trade less frequently and in smaller volumes than more widely held securities. Some securities of smaller issuers may be illiquid or may be restricted as to resale, and their values may have significant volatility. The Fund may be unable to liquidate its posi tions in such securities at any time, or at a favorable price, in order to meet the Fund's obligations. Returns on investments in securities of small-, mid- and micro-capitalization companies could trail the returns on investments in securities of larger companies.
State Street Small_Mid Cap Equity Index Fund | Financial Sector Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Financial Sector Risk: Financial services companies are subject to extensive governmental regulation, which may limit both the amounts and types of loans and other financial commitments they can make, the interest rates and fees they can charge, the scope of their activities, the prices they can charge and the amount of capital they must maintain. Profitability is largely dependent on the availability and cost of capital funds and can fluctuate significantly when interest rates change or due to increased competition. In addition, deterioration of the credit markets generally may cause an adverse impact in a broad range of markets, including U.S. and international credit and interbank money markets generally, thereby affecting a wide range of financial institutions and markets. Certain events in the financial sector may cause an unusually high degree of volatility in the financial markets, both domestic and foreign, and cause certain financial services companies to incur large losses. Securities of financial services companies may experience a dramatic decline in value when such companies experience substantial declines in the valuations of their assets, take action to raise capital (such as the issuance of debt or equity securities), or cease operations. Credit losses resulting from financial difficulties of borrowers and financial losses associated with investment activities can negatively impact the sector. Insurance companies may be subject to severe price competition. Adverse economic, business or political developments could adversely affect financial institutions engaged in mortgage finance or other lending or investing activities directly or indirectly connected to the value of real estate.
State Street Small_Mid Cap Equity Index Fund | Industrial Sector Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Industrial Sector Risk: Industrial companies are affected by supply and demand both for their specific product or service and for industrial sector products and services in general. Government regulation, world events, exchange rates and economic conditions, technological developments and liabilities for environmental damage and general civil liabilities will likewise affect the performance of these companies. Aerospace and defense companies, a component of the industrial sector, can be significantly affected by government spending policies because companies involved in this industry rely, to a significant extent, on U.S. and foreign government demand for their products and services. Thus, the financial condition of, and investor interest in, aerospace and defense companies are heavily influenced by governmental defense spending policies which are typically under pressure from efforts to control the U.S. (and other) government budgets. Transportation securities, another component of the industrial sector, are cyclical and have occasional sharp price movements which may result from changes in the economy, fuel prices, labor agreements and insurance costs.
State Street Small_Mid Cap Equity Index Fund | Information Technology Sector Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Information Technology Sector Risk: Market or economic factors impacting information technology companies could have a major effect on the value of the Fund's investments. The value of stocks of information technology companies is particularly vulnerable to rapid changes in technology product cycles, rapid product obsolescence, government regulation and competition, both domestically and internationally, including competition from foreign competitors with lower production costs. Like other technology companies, information technology companies may have limited product lines, markets, financial resources or personnel. Stocks of technology companies and companies that rely heavily on technology, especially those of smaller, less-seasoned companies, tend to be more volatile than the overall market. Information technology companies are heavily dependent on patent and intellectual property rights, the loss or impairment of which may adversely affect profitability. Additionally, companies in the information technology sector may face dramatic and often unpredictable changes in growth rates and competition for the services of qualified personnel.
State Street Small_Mid Cap Equity Index Fund | Health Care Sector Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Healthcare Sector Risk: Companies in the healthcare sector are subject to extensive government regulation and their profitability can be significantly affected by restrictions on government reimbursement for medical expenses, rising costs of medical products and services, pricing pressure (including price discounting), limited product lines and an increased emphasis on the delivery of healthcare through outpatient services. Companies in the healthcare sector are heavily dependent on obtaining and defending patents, which may be time consuming and costly, and the expiration of patents may also adversely affect the profitability of these companies. Healthcare companies are also subject to extensive litigation based on product liability and similar claims. In addition, their products can become obsolete due to industry innovation, changes in technologies or other market developments. Many new products in the healthcare sector require significant research and development and may be subject to regulatory approvals, all of which may be time consuming and costly with no guarantee that any product will come to market.
State Street Small_Mid Cap Equity Index Fund | Indexing Strategy Index Tracking Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Indexing Strategy/Index Tracking Risk: The Fund is managed with an indexing investment strategy, attempting to track the performance of an unmanaged index of securities, regardless of the current or projected performance of the Index or of the actual securities comprising the Index. This differs from an actively-managed fund, which typically seeks to outperform a benchmark index. As a result, the Fund's performance may be less favorable than that of a portfolio managed using an active investment strategy. The structure and composition of the Index will affect the performance, volatility, and risk of the Index and, consequently, the performance, volatility, and risk of the Fund. While the Adviser seeks to track the performance of the Index (i.e., achieve a high degree of correlation with the Index), the Fund's return may not match the return of the Index. The Fund incurs a number of operating expenses not applicable to the Index, and incurs costs in buying and selling securities. In addition, the Fund may not be fully invested at times, generally as a result of cash flows into or out of the Fund or reserves of cash held by the Fund to meet redemptions. The Adviser may attempt to replicate the Index return by investing in fewer than all of the securities in the Index, or in some securities not included in the Index, potentially increasing the risk of divergence between the Fund's return and that of the Index.
State Street Small_Mid Cap Equity Index Fund | Consumer Discretionary Sector Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Consumer Discretionary Sector Risk: The success of consumer product manufacturers and retailers is tied closely to the performance of the overall global economy, interest rates, competition and consumer confidence. Success depends heavily on disposable household income and consumer spending. Also, companies in the consumer discretionary sector may be subject to severe competition, which may have an adverse impact on their respective profitability. Changes in demographics and consumer tastes can also affect the demand for, and success of, consumer products and services in the marketplace.
State Street Small_Mid Cap Equity Index Fund | Counterparty Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Counterparty Risk: The Fund will be subject to credit risk with respect to the counterparties with which the Fund enters into derivatives contracts, repurchase agreements, reverse repurchase agreements, and other transactions. If a counterparty fails to meet its contractual obligations, the Fund may be unable to terminate or realize any gain on the investment or transaction, or to recover collateral posted to the counterparty, resulting in a loss to the Fund. If the Fund holds collateral posted by its counterparty, it may be delayed or prevented from realizing on the collateral in the event of a bankruptcy or insolvency proceeding relating to the counterparty.
State Street Small_Mid Cap Equity Index Fund | Derivatives Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Derivatives Risk: Derivative transactions can create investment leverage and may have significant volatility. It is possible that a derivative transaction will result in a much greater loss than the principal amount invested, that changes in the value of a derivative transaction may not correlate perfectly with the underlying asset, and that the Fund may not be able to close out a derivative transaction at a favorable time or price. The counterparty to a derivatives contract may be unable or unwilling to make timely settlement payments, return the Fund's margin, or otherwise honor its obligations. A derivatives transaction may not behave in the manner anticipated by the Adviser or may not have the effect on the Fund anticipated by the Adviser.
State Street Small_Mid Cap Equity Index Fund | Large Transactions Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Large Transactions Risk: To the extent a large proportion of the shares of the Portfolio or Fund are held by a small number of shareholders (or a single shareholder), including funds or accounts over which the Adviser has investment discretion, the Fund is subject to the risk that these shareholders will purchase or redeem Portfolio or Fund shares in large amounts rapidly or unexpectedly, including as a result of an asset allocation decision made by the Adviser. In addition, a large number of shareholders collectively may purchase or redeem Portfolio or Fund shares in large amounts rapidly or unexpectedly (collectively, such transactions are referred to as “large shareholder transactions”). Large shareholder transactions could adversely affect the ability of the Portfolio or Fund to conduct its investment program.
State Street Small_Mid Cap Equity Index Fund | Liquidity Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Liquidity Risk: Lack of a ready market, stressed market conditions, or restrictions on resale may limit the ability of the Fund to sell a security at an advantageous time or price or at all. Illiquid investments may trade at a discount from comparable, more liquid investments and may be subject to wide fluctuations in market value. Illiquidity of the Fund's holdings may also limit the ability of the Fund to obtain cash to meet redemptions on a timely basis. In addition, the Fund, due to limitations on investments in any illiquid investments and/or the difficulty in purchasing and selling such investments, may be unable to achieve its desired level of exposure to a certain market or sector.
State Street Small_Mid Cap Equity Index Fund | Master Feeder Structure Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Master/Feeder Structure Risk: The Fund pursues its objective by investing substantially all of its assets in another pooled investment vehicle (a “master fund”). The ability of the Fund to meet its investment objective is directly related to the ability of the master fund to meet its investment objective. The Adviser serves as investment adviser to the master fund, leading to potential conflicts of interest. The Fund will bear its pro rata portion of the expenses incurred by the master fund. Substantial redemptions by other investors in a master fund may affect the master fund's investment program adversely and limit the ability of the master fund to achieve its objective.
State Street Small_Mid Cap Equity Index Fund | Risk of Investment in Other Pools [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Risk of Investment in Other Pools: If the Fund invests in another pooled investment vehicle, it is exposed to the risk that the other pool will not perform as expected and is exposed indirectly to all of the risks applicable to an investment in such other pool. The investment policies of the other pool may not be the same as those of the Fund; as a result, an investment in the other pool may be subject to additional or different risks than those to which the Fund is typically subject. The Fund bears its proportionate share of the fees and expenses of any pool in which it invests. The Adviser or an affiliate may serve as investment adviser to a pool in which the Fund may invest, leading to potential conflicts of interest. It is possible that other clients of the Adviser or its affiliates will purchase or sell interests in a pool sponsored or managed by the Adviser or its affiliates at prices and at times more favorable than those at which the Fund does so.
State Street Small_Mid Cap Equity Index Fund | Unconstrained Sector Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Unconstrained Sector Risk: The Fund may invest a substantial portion of its assets within one or more economic sectors or industries, which may change from time to time. Greater investment focus on one or more sectors or industries increases the potential for volatility and the risk that events negatively affecting such sectors or industries could reduce returns, potentially causing the value of the Fund's Shares to decrease, perhaps significantly.
State Street Small_Mid Cap Equity Index Fund | Valuation Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Valuation Risk: Certain portfolio holdings may be valued on the basis of factors other than market quotations. This may occur more often in times of market turmoil or reduced liquidity. There are multiple methods that can be used to value a portfolio holding when market quotations are not readily available. The value established for any portfolio holding at a point in time might differ from what would be produced using a different methodology or if it had been priced using market quotations. Portfolio holdings that are valued using techniques other than market quotations, including “fair valued”securities, may be subject to greater fluctuation in their valuations from one day to the next than if market quotations were used. In addition, there is no assurance that the Fund could sell or close out a portfolio position for the value established for it at any time, and it is possible that the Fund would incur a loss because a portfolio position is sold or closed out at a discount to the valuation established by the Fund at that time. Investors who purchase or redeem Fund Shares on days when the Fund is holding fair-valued investments may receive fewer or more shares or lower or higher redemption proceeds than they would have received if the Fund had not fair-valued the holding(s) or had used a different valuation methodology.
State Street Small_Mid Cap Equity Index Fund | Class A  
Risk Return Abstract rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice 5.25%
Maximum Deferred Sales Charge (Load) (as a percentage of the lower of the sale proceeds or the original offering price) rr_MaximumDeferredSalesChargeOverOther none [1]
Management Fee rr_ManagementFeesOverAssets 0.03%
Distribution and/or Shareholder Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other Expenses rr_OtherExpensesOverAssets 0.31% [2]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.59%
Less Fee Waivers and/or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.095%) [3]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements rr_NetExpensesOverAssets 0.495%
1 year rr_ExpenseExampleYear01 $ 573
3 years rr_ExpenseExampleYear03 695
5 years rr_ExpenseExampleYear05 828
10 years rr_ExpenseExampleYear10 $ 1,216
2016 rr_AnnualReturn2016 15.67%
2017 rr_AnnualReturn2017 17.87%
2018 rr_AnnualReturn2018 (9.35%)
2019 rr_AnnualReturn2019 27.42%
2020 rr_AnnualReturn2020 31.79%
2021 rr_AnnualReturn2021 11.95%
2022 rr_AnnualReturn2022 (25.74%)
2023 rr_AnnualReturn2023 24.60%
2024 rr_AnnualReturn2024 16.81%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel <span style="font-family:Arial;font-size:9.02pt;font-weight:bold;margin-left:0.0pt;">Highest Quarterly Return</span>
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2020
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 30.41%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel <span style="font-family:Arial;font-size:9.02pt;font-weight:bold;margin-left:0.0pt;">Lowest Quarterly Return</span>
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2020
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (28.02%)
OneYear rr_AverageAnnualReturnYear01 10.68%
FiveYears rr_AverageAnnualReturnYear05 8.60%
SinceInception rr_AverageAnnualReturnSinceInception 9.85%
Inception Date rr_AverageAnnualReturnInceptionDate Oct. 15, 2015
State Street Small_Mid Cap Equity Index Fund | Class I  
Risk Return Abstract rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (Load) (as a percentage of the lower of the sale proceeds or the original offering price) rr_MaximumDeferredSalesChargeOverOther none
Management Fee rr_ManagementFeesOverAssets 0.03%
Distribution and/or Shareholder Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.31% [2]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.34%
Less Fee Waivers and/or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.095%) [3]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements rr_NetExpensesOverAssets 0.245%
1 year rr_ExpenseExampleYear01 $ 25
3 years rr_ExpenseExampleYear03 100
5 years rr_ExpenseExampleYear05 181
10 years rr_ExpenseExampleYear10 $ 421
OneYear rr_AverageAnnualReturnYear01 17.10%
FiveYears rr_AverageAnnualReturnYear05 10.06%
SinceInception rr_AverageAnnualReturnSinceInception 10.57%
Inception Date rr_AverageAnnualReturnInceptionDate Oct. 15, 2015
State Street Small_Mid Cap Equity Index Fund | Class K  
Risk Return Abstract rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (Load) (as a percentage of the lower of the sale proceeds or the original offering price) rr_MaximumDeferredSalesChargeOverOther none
Management Fee rr_ManagementFeesOverAssets 0.03%
Distribution and/or Shareholder Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.11% [2]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.14%
Less Fee Waivers and/or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.095%) [3]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements rr_NetExpensesOverAssets 0.045%
1 year rr_ExpenseExampleYear01 $ 5
3 years rr_ExpenseExampleYear03 35
5 years rr_ExpenseExampleYear05 69
10 years rr_ExpenseExampleYear10 $ 170
OneYear rr_AverageAnnualReturnYear01 17.19%
FiveYears rr_AverageAnnualReturnYear05 10.26%
SinceInception rr_AverageAnnualReturnSinceInception 9.94%
Inception Date rr_AverageAnnualReturnInceptionDate Aug. 11, 2015
State Street Small_Mid Cap Equity Index Fund | Return After Taxes on Distributions | Class A  
Risk Return Abstract rr_RiskReturnAbstract  
OneYear rr_AverageAnnualReturnYear01 10.04%
FiveYears rr_AverageAnnualReturnYear05 7.04%
SinceInception rr_AverageAnnualReturnSinceInception 8.30%
State Street Small_Mid Cap Equity Index Fund | Return After Taxes on Distributions and Sale of Fund Shares | Class A  
Risk Return Abstract rr_RiskReturnAbstract  
OneYear rr_AverageAnnualReturnYear01 6.47%
FiveYears rr_AverageAnnualReturnYear05 6.43%
SinceInception rr_AverageAnnualReturnSinceInception 7.44%
State Street Small_Mid Cap Equity Index Fund | S&P 500 Index (reflects no deduction for fees, expenses or taxes)  
Risk Return Abstract rr_RiskReturnAbstract  
OneYear rr_AverageAnnualReturnYear01 25.02%
FiveYears rr_AverageAnnualReturnYear05 14.53%
SinceInception rr_AverageAnnualReturnSinceInception 13.72%
State Street Small_Mid Cap Equity Index Fund | Russell Small Cap Completeness Index (reflects no deduction for fees, expenses or taxes)  
Risk Return Abstract rr_RiskReturnAbstract  
OneYear rr_AverageAnnualReturnYear01 17.14%
FiveYears rr_AverageAnnualReturnYear05 10.27%
SinceInception rr_AverageAnnualReturnSinceInception 9.99%
[1] A contingent deferred sales charge (“CDSC”) of 1.00% may be assessed on redemptions of Class A shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.
[2] Other Expenses have been restated to reflect current fees.
[3] The Fund's investment adviser, SSGA Funds Management, Inc. (the “Adviser”or “SSGA FM”), is contractually obligated until April 30, 2026, separately with respect to each of the Fund and the Portfolio, (i) to waive up to the full amount of the advisory fee payable by the Fund or the Portfolio, and/or (ii) to reimburse the Fund or the Portfolio to the extent that Total Annual Fund Operating Expenses (exclusive of non-recurring account fees, interest, taxes, extraordinary expenses, and distribution, shareholder servicing, and sub-transfer agency fees) exceed 0.045% of the Fund's and 0.03% of the Portfolio's average daily net assets on an annual basis (for avoidance of doubt, the waiver and/or reimbursement is inclusive of the allocation of expenses from the Portfolio). This waiver and/or reimbursement may not be terminated prior to April 30, 2026 except with approval of the Fund's/Portfolio's Board of Trustees.